Field of the Invention
The present invention relates to electro-photographic image forming apparatuses, such as copiers, printers, facsimiles and multi-function printers.
Description of the Related Art
Hitherto, image forming apparatuses have adopted charging rollers, having a relatively low application voltage and those can be easily downsized, as primary charging units for charging photosensitive drums. A superimposed voltage (Vdc+Vac) having superimposed a DC voltage Vdc and an AC voltage Vac is applied from a charging high voltage power supply to the charging roller as charging voltage. When an AC voltage Vac (peak-to-peak voltage) that is equal to or higher than twice the amount of a discharge starting voltage Vth is applied to the charging roller, a surface potential Vd of the photosensitive drum is converged to a potential of the DC voltage Vdc. That is, the photosensitive drum is charged uniformly to a desired surface potential.
The discharge starting voltage Vth between the charging roller and the photosensitive drum may be fluctuated due to a material of the charging roller, a film thickness of a photosensitive layer of the photosensitive drum, a rotational speed (process speed) of the photosensitive drum, environments (such as temperature) within the apparatus body, and so on. If the AC voltage Vac applied to the charging roller remains unchanged when the discharge starting voltage Vth is fluctuated, the photosensitive drum may not be charged uniformly to the desired surface potential. An image forming apparatus where an appropriate AC voltage Vac is computed and applied to the charging roller, in order to uniformly charge the photosensitive drum to the desired surface potential even when the discharge starting voltage Vth is fluctuated, is known. For example, an apparatus that performs discharge current control by applying a plurality of AC voltages Vac sequentially, and based on the change of alternating currents Idc flowing to the charging roller, computing the AC voltage Vac to be applied to charge the photosensitive drum is proposed (Japanese Patent Application Laid-Open Publication No. 2001-201920).
Further, an image forming apparatus having a surface electrometer within an apparatus body, actually measuring a surface potential Vd of a photosensitive drum using the surface electrometer, and changing an AC voltage Vac based on the measured result is proposed (Japanese Patent Application Laid-Open Publication No. H07-44063).
As described above, the discharge starting voltage Vth is influenced by the temperature within the apparatus body, for example. That is, an electrical resistance of the charging roller differs between a case where the temperature within the apparatus body is a normal temperature (such as 15 through 25° C.) and a case where the temperature is a low temperature (such as 0 through 15° C.). Generally, in a low temperature environment, the resistance of the charging roller is increased and current flow is not likely to flow compared to normal temperature environment, so that the discharge starting voltage Vth is increased. Therefore, the apparatus detects the temperature within the apparatus body through a temperature sensor, and performs respective discharge current controls for normal temperature and for low temperature, based on the detected temperature. In the respective discharge current controls for normal temperature and for low temperature, the setting of voltages of the AC voltage Vac applied to the charging roller differ.
However, the temperature sensor is often provided at a location distant from the charging roller within the apparatus body, and the temperature detected by the temperature sensor does not always precisely reflect the temperature in the vicinity of the charging roller. That is, the temperature detected by the temperature sensor is merely the temperature measured at a specific location within the apparatus body, so that the detected temperature may not correspond to the temperature in the vicinity of the charging roller. In that case, even if discharge current control is performed, an AC voltage Vac double or lower the amount of discharge starting voltage Vth may be charged to the charging roller. As a result, the surface potential Vd of the photosensitive drum will not be converged to the potential of the DC voltage Vdc, and a fogging phenomenon may occur. It may be possible to adopt an apparatus having a surface electrometer with the aim to prevent the surface potential Vd of the photosensitive drum from not being converted to the potential of the DC voltage Vdc, but an apparatus equipped with a surface electrometer cannot easily be downsized, and related costs are expensive, so that it is not desirable to adopt such apparatus.